Feeding of certain driven members, particularly computer forms, requires a feed mechanism capable of moving at extremely high speeds and of undergoing rapid acceleration and deceleration. Any skew or meandering of the drive causes misalignment of the forms or members with operating devices, such as printer heads. If the feed mechanism is not fast acting, misregistration can readily occur. In the case of perforated documents which are fed through pin drive elements, skewing and improper entrance and stripping of the pin tends to tear the document, jam the document in the mechanism and cause costly down time of the entire computer system. These requirements for high speed rapid acceleration and deceleration, close tolerances on skewing, entry and exit of the pins, an accuracy of movement with respect to the operating member, such as the printer head, are not compatible with each other and have resulted in the use of complex and costly feed mechanisms. Characteristic of these feed mechanisms is the so called "tractor" which employs a chain or belt which is carried by a pair of sprocket gears, each link of the chain or portion of the belt carrying a sprocket pin which extends from the chain or belt. The pins enter the perforations in the document as they pass from curved paths around the sprockets into a straight path and are stripped from the documents as they leave the curved path when drawn around a second sprocket. Close tolerances are required between the pin location and the sprockets or else pairs of these tractors along opposite marginal edges of the document will not be in synchronism and will cause the entire document to skew, drag, bind, or actually tear in the mechanism. Thus, complex phasing adjustments at the sprocket shafts are required and must be maintained by regular adjustments during operation. Such adjustments are oftentimes incompatible with clean entry and stripping of the pins which cause driving forces to be applied to the document perforations in other than the linear path, thus tearing and damaging the document.
An improved feed mechanism which overcomes the aforementioned difficulties and disadvantages is disclosed in U.S. Pat. No. 3,825,162. The patent discloses a feed mechanism utilizing a strip of flexible material which is formed into an endless belt and presents a feeding surface facing outwardly and a surface opposite to the feeding surface which faces the area enclosed by the belt. Drive elements project from the feeding surface of the belt and may be in the form of pins which are spaced equidistant from each other; the increments between the pins being equal to the increments between the perforations of the document which is adapted to be driven by the mechanism. The drive elements may also have portions which project from the opposite or inner surface of the belt and which may be integral with the portions of the element which project from the feeding surface thereof. A frame supports the belt along the opposite or inner surface thereof and provides a bed along a linear path which may be disposed adjacent to the document to be fed. Driving means which may be disposed in the frame are provided for driving the belt. These driving means may be sprockets, at least one of which is a drive sprocket, while the other is an idler sprocket. The sprockets may be formed with hemi-cylindrical slots which engage the potions of the drive elements projecting from the opposite surface of the belt. These portions may be hemi-cylindrical and function as rollers in that as the sprockets turn, the drive elements roll with respect to the hemi-cylindrical slots in the sprockets.
Although the feed mechanism described in the aforementioned patent, in addition to driving and aligning the document, provides tension to the document as it travels through to the printing area, additional means may be necessary for proper control of the tension.
A typical prior art device for providing the additional tensioning means is shown in U.S. Pat. No. 3,746,142. As described in this patent, a paper tension mechanism for a line printer having an output tractor drive with a tension mechanism being utilized in lieu of an input tractor drive is provided. A cross shaft has pinwheels secured near each end thereof to engage with perforated paper, and one end of the shaft carries a slip bearing and a torsion collar with a torsion spring therebetween, the spring being in contact with the bearing and the collar, and wherein the slip bearing is pinched between a fixed block and a spring loaded block. Rotation of the cross shaft winds the torsion spring until the force is sufficient to rotate the slip bearing. The "drag" provided by the bearing applies tension to the paper in the area between the output tractors and the pinwheels, and when paper movement is stopped, as during the printing operation, the torsion spring tends to unwind and thereby holds the paper in tension. Adjustment means is also provided for the slip bearing by either increasing or decreasing the amount of pinch of the split block on the bearing, which, in turn, affects the tension in the paper. Although it is not clear when this adjustment step occurs, it appears that it is accomplished only when the paper is at rest, i.e., a static adjustment can only be performed which reduces the printing rate. Further, the relationship between the increase/decrease of paper tension versus the rotation of the adjustment means is clearly not linear which may impact the accuracy of the adjustment. Further, the use of spring force to adjust tension has several disadvantages including unpredictable friction and paper thickness, each a major problem in itself.